1. Field of the Invention
The present invention relates to an image capturing apparatus and a control method therefor, and more specifically, to an image capturing apparatus having autofocus function and a control method therefor.
2. Description of the Related Art
Typical schemes of focus control methods for image capturing apparatuses include a contrast AF scheme and a phase difference AF scheme. The contrast AF scheme and the phase difference AF scheme are AF schemes widely used in video cameras and digital still cameras, with an image sensor being used as a focus detection sensor.
In these focus control methods, focus detection results may contain errors due to various aberrations of an optical system. Various methods have been proposed to reduce such errors. Japanese Patent Laid-Open No. 2007-94236 discloses a method in which frequency components are extracted from a signal for focus detection in the two directions that are orthogonal to each other and corrected, and focus detection is performed based on an added output of the corrected frequency components.
Such focus detection error occurs depending on the arrangement direction of pixels on the image sensor that output signals to be used for focus control in the contrast AF method and the phase difference AF method regardless of the focus control methods in a case where an image capturing apparatus using an optical system that has astigmatism, for example.
However, in the configuration disclosed in Japanese Patent Laid-Open No. 2007-94236, there is a problem in which a focus detection error cannot be sufficiently corrected as will be explained below. Firstly, according to Japanese Patent Laid-Open No. 2007-94236, in order to reduce the focus detection error, the focus detection is performed by setting the evaluation directions of the signals for focus control to the horizontal and vertical directions, and weighing the focus detection results detected in the respective evaluation directions. On the other hand, the focus detection error is decided by an angle of an edge of a subject, not by the evaluation direction. For example, if 45° is only an existing edge angle of the subject, a focus detection error is the same for the evaluation in the horizontal direction and for the evaluation in the vertical direction. However, Japanese Patent Laid-Open No. 2007-94236 is silent about a method for correcting the focus detection error in accordance with the edge angle of the subject.